int rawIndex

Returns the starting position of the current rune in the string.

Returns null if the current rune is null.

Source 

int get rawIndex => (_position != _nextPosition) ? _position : null;
void rawIndex=(int rawIndex)

Resets the iterator to the rune at the specified index of the string.

Setting a negative rawIndex, or one greater than or equal to string.length, is an error. So is setting it in the middle of a surrogate pair.

Setting the position to the end of then string will set current to null.

Source 

void set rawIndex(int rawIndex) {
  RangeError.checkValidIndex(rawIndex, string, "rawIndex");
  reset(rawIndex);
  moveNext();
}